Technology traditionally relating to OFDM modulation mobile communication systems has been disclosed in Unexamined Japanese Patent Publication No. HEI 11-17643.
FIG. 1 is a block diagram showing the configuration of a conventional transmitting apparatus and receiving apparatus. The transmitting apparatus and receiving apparatus in FIG. 1 are provided, respectively, in a base station apparatus and communication terminal apparatus in an OFDM modulation mobile communication system.
The transmitting apparatus 10 mainly consists of an S/P (Serial/Parallel) conversion section 11, an IFFT (Inverse Fast Fourier Transform) section 12, and a modulation section 13. The receiving apparatus 20 mainly consists of a reception filter section 21, an FFT (Fast Fourier Transform) section 22, and a P/S (Parallel/Serial) conversion section 23.
With this kind of configuration, transmit data undergoes parallel conversion to N (where N is a natural number) subcarriers by the S/P conversion section 11 in the transmitting apparatus 10, then undergoes an inverse Fourier transform together with 2M (where M is a natural number) 0 data units by the IFFT section 12, and after being modulated by the modulation section 13, is transmitted as a radio signal from an antenna.
A radio signal transmitted from the transmitting apparatus 10 is received by the antenna of the receiving apparatus 20, and a signal of a predetermined frequency is passed by the reception filter section 21, is demodulated by undergoing a Fourier transform by the FFT section 22, and is converted to serial form by the P/S conversion section 23. By this means, receive data is obtained.
FIG. 2 is a frequency spectrum diagram for the conventional OFDM modulation method. As shown in FIG. 2, with the OFDM modulation method, a signal transmission band 51 is provided in which there is a plurality of carrier frequency signals f1 to fn. In the signal transmission band 51, adjacent carrier frequency signals are arranged so that—with f1 and f2, for example—the 0 level point of one carrier frequency signal f2 overlaps the peak level point of the other carrier frequency signal f1 on the same frequency axis.
Also, with the OFDM modulation method, the frequency intervals at which a peak level point and 0 level point overlap are equal intervals, being a frequency interval which is the inverse of the symbol rate. For example, with a symbol rate of 1 μsec , the frequency interval is every 1 MHz.
Also, guard frequency bands 52 and 53—bands in which a carrier is not used—are provided on either side of the signal transmission band 51. This is done to suppress power leakage outside band 51 and prevent influence on other communications.
However, in a mobile communication system that uses the conventional OFDM modulation method, the fact that guard frequency bands 52 and 53 are not used for signal transmission presents a problem in that spectrum efficiency is reduced proportionately.
Moreover, with the OFDM modulation method, there is a problem in that the amplitude (peak) at a point at which the same carrier frequency signals overlap is large with respect to the average power, and as a countermeasure to this, a small number of carriers are allocated for peak suppression in a conventional mobile communication system. However, when peak suppression carriers are provided, there is a problem of a further drop in spectrum efficiency.